It is well known that the electrical sparks fed to the spark plugs of an internal combustion engine are conventionally produced by means of an ignition coil having its high voltage secondary winding connected to the engine's spark plugs through a distributor, and having its low voltage primary winding connected to a low voltage source, typically a 12 volt battery or an alternator system driven by the engine. An engine driven switching device, typically a mechanical contact breaker, produces interruptions in the current flowing in the coil's primary winding and consequently high voltage pulses are produced in the coil's secondary winding, which are applied to the spark plugs.
Recently, a proposal has been made to increase the energy of the sparks applied to the spark plugs, by connecting a capacitor to the primary coil of the winding, charging the capacitor to a voltage much higher than the conventional 12 volt supply voltage from the engine's battery and alternator, and discharging the capacitor through the coil's primary winding each time a spark is required. With such a capacitive discharge system, the total spark energy for each firing of a cylinder of the engine, is increased substantially with respect to the conventional spark ignition apparatus, but the duration of the sparks produced by the system is much less than those produced by the conventional apparatus. Such shorter sparks can prove disadvantageous with certain engines, since the sparks may not produce a complete ignition of the fuel/air mixture.
It has been proposed in British Patent Specification No. 1,427,600 to Hitachi Ltd. to provide a combination of the aforementioned two types of ignition system, which results in a system in which the sparks are initiated by the relatively short high energy pulses from the capacitor discharge system, and the sparks are maintained after completion of the capacitor discharge by the lower energy longer duration pulses from the conventional system.
The Hitachi arrangement however suffers from the disadvantage that the system is essentially a pulsed system in which pulses are inductively coupled through ignition coils to the spark plugs, which limits the energy that can be supplied to sustain the spark, and also imposes a limit on the maximum duration of the spark. With this pulsed system, the current flowing in the arc established between the spark plug's electrodes necessarily gradually reduces towards zero towards the end of the period of the spark, which limits the total spark energy that can be injected by the pulsed system into the spark.